In recent years, as visual telephones, electronic still cameras, and like are widely used, an image digital processing technique, especially a high-efficiency encoding technique for compressing image data, is progressing.
For example, an image data compressing apparatus using the high-efficiency encoding technique has an arrangement as follows. Input image data is divided into a plurality of data blocks by a block division circuit. One block is constituted by data of, e.g., 8 horizontal pixels.times.8 vertical pixels=64 pixels. Block image data is input to a DCT (Discrete Cosine Transformation) circuit to be transformed into DCT coefficients representing frequency components. A DCT coefficient output from the DCT circuit is input to a quantization circuit to be quantized. A quantization output is input to a variable length encoding circuit to be encoded by utilizing Haffman codes. An encoded output is input to a buffer memory. Output information from the buffer memory is output to a transmission path, e.g., a telephone circuit, or a recording medium, e.g., a memory card. The buffer memory is provided to control the amount of output information. A change in information amount in the buffer memory is monitored by a quantization control circuit. Upon reception of information representing a change in information amount in the buffer memory, the quantization control circuit switches the quantization coefficients of the quantization circuit and performs control as required so the output information amount falls within a predetermined range.
The buffer memory and the quantization control circuit are provided due to the following reason. When an image is compressed by the system described above in accordance with variable length encoding, generally, an image constituted by fine patterns corresponds to a quantization output and an encoded output each having a large bit count, and an image constituted by a monotonous pattern corresponds to a quantization output and an encoded output each having a small bit count. If the quantization control circuit is not provided, the transmission rate becomes excessive, or the capacity of the recording medium becomes short.
In the conventional image data compressing apparatus, the quantization characteristics are controlled for the sole purpose of constantly limiting the output information amount to fall within the predetermined range.
On the other hand, however, since the quantization characteristics are constantly controlled, when an encoded image signal is decoded, the image quality is degraded. For example, assume that an image of a frame, an upper half of which is constituted by a monotonous pattern and a lower half of which is constituted by fine patterns, is to be quantized. When the image data of the upper half region is to be quantized, an excessive bit count is assigned to it; when the image data of the lower half region is to be quantized, an insufficient bit count is assigned to it. Thus, the image quality is degraded. In other words, the image quality differs between the upper and lower regions of the frame, reproducing an unnatural image.